Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.

Basis for Assessment

The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to a recent increase on chlorophyll concentrations that suggest the potential for algae-related aesthetically objectionable conditions at this lake.   The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts in the lake.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.   The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002).  

EXPLANATION:  Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Volga Lake are "fully supported / threatened" due to the somewhat elevated levels of chlorophyll at this lake.   Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 66, 60, and 47, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the upper boundary of eutrophic lakes; and the index value for secchi depth is in the upper range of mesotrophic lakes.   These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is relatively low and water transparency is exceptional for an Iowa lake.   These results suggest that this lake does not have impairments due to aesthetically objectionable blooms of algae although levels of chlorophyll are sufficiently high to suggest a threat to the full support of the Class A uses.   According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with a relatively low values for chlorophyll-a indicates that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae.   The results of ISU monitoring suggest that zooplankton grazing of algae most likely functions as the primary non-phosphorus limitation on algal production at this lake.   Neither nitrogen limitation nor inorganic turbidity appear to influence algal production.   The presence of relatively large populations of zooplankton at Volga Lake that graze on algae may explain, in part, the discrepancy between the TSI value for phosphorus and those for chlorophyll-a and secchi depth.   Downing et al.  (2002) show that Cladoceran taxa (e.g., Daphnia) comprised approximately 15% of the dry mass of the zooplankton community in the early August sample and 45% of the mid-September sample.   The summer 2000 average mass of Cladocerans at this lake (26.6 mg/l) was 37th highest of the 131 lakes sampled, thus suggesting the potential for zooplankton grazers to limit algal production.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Volga Lake is 44.   This ratio is relatively high and does not suggest any limitations to algal production due to nitrogen availability.   This relatively high ratio does suggest an excess of total nitrogen at this lake (Volga Lake had the 18th highest 2000-02 median total nitrogen concentration of the 131 lakes sampled).   The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing designated uses.   The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Volga Lake was 1.2 mg/l.   This median level is the 8th lowest of the 130 lakes sampled and thus does not suggest any influence on algal production or any contributions to in-lake turbidity.   Thus, the water quality conditions at this lake suggest that the Class A uses are “fully supported / threatened” due to the somewhat elevated levels of chlorophyll-a that suggest a potential water quality problem with aesthetically objectionable conditions due to algal blooms.   This assessment differs from that developed for the 2002 report (Class A uses “fully supported”) due to a relatively large increase in the TSI values for chlorophyll-a:  the TSI value based on data from ISU monitoring in 2000 and 2001 was 48 (extremely low chlorophyll); the TSI based on all three years of data (2000-02) was 60 (somewhat elevated chlorophyll).   This increase in TSI values reflects the consistently higher (by an order of magnitude) chlorophyll-a concentrations at Volga lake in 2002.

This lake does not appear to have problems due to presence of nuisance algal species (i.e., bluegreen algae).   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake.   Sampling in 2000 showed that bluegreen algae comprised approximately 30% of the wet mass of the phytoplankton community; this percentage declined to approximately 10% by the mid-September sampling.   The summer 2000 average mass of bluegreen algae at this lake (3.3 mg/l) was the 33rd lowest of the 131 lakes sampled, thus suggesting low populations and minimal impacts from this nuisance aquatic algae.  

The slightly eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to siltation impacts in the lake.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.